Influence of Turbidity Currents Upon Basin Waters

Influence of Turbidity Currents Upon Basin WatersAuthor(s): K. O. Emery, Jobst Hulsemann and K. S. RodolfoSource: Limnology and Oceanography, Vol. 7, No. 4 (Sep., 1962), pp. 439-446Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2832842 .

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LIMNOLOGY AND

OCEkANOGRAPHY

October, 1962

VOLUME VII

NUMBER 4

INFLUENCE OF TURBIDITY CURRENTS UPON BASIN WATERS

K. 0. Emery,1 Jobst Hfilsemann,1 and K. S. Rodolfo University of Southern California

ABSTRACT

Waters at the sill depths and near the bottoms of two basins off southern California were sampled in a grid pattern and analyzed for possible anomalies in temperature, salinity, and contents of oxygen, nutrients and suspended sediment due to turbidity currents. The only significantly anomalous patterns revealed were those of low salinity and high suspended sediment, atop a turbidity-current apron opposite the mouths of two submarine canyons. This water was probably emplaced by turbidity currents, and its presence here indicates that areal variations of bottom-water properties elsewhere warrant detailed studies.

INTRODUCTION

The roles of turbidity currents in shaping the topography of the sea floor and in de- positing coarse-grained sediments are be- coming well known. Less understood are their effects upon benthic and bathypelagic life, and as yet unestablished are their modi- fications of bottom waters. The basins off southern California may serve as convenient areas for detecting and measuring these aspects of turbidity currents.

Santa Monica and San Pedro basins were chosen for testing because of their nearness to the coast and to harbor facilities. Each basin is reached by submarine canyons whose heads are so close to shore that they are able to intercept much of the sediment caused to drift along the littoral zone by diagonal wave approach (Emery 1960a, pp. 28, 118, 219). Intermittent sliding of the sediment down the canyon axes evi- dently produces turbidity currents which deposit graded sands in broad thin lenses at the canyon mouths and beyond. Long-

1 Present address Woods Hole Oceanographic Institution.

continued deposition of the sands has formed a 35-km concave slope at the northwest end of Santa Monica Basin and a smaller one at the north side of San Pedro Basin (Emery 1960b) (Fig. 1). Only the finest sediment reaches the extremely flat basin floors beyond the toes of these slopes. In these basins the thickness of the turbidity- current sediment averages approximately half that of hemipelagic (grain-by-grain) sediments (Fig. 1), and radiocarbon dating of 6 cores shows that total sediment is deposited at an average rate of 70 cm/1,000 years.

Radiocarbon dating also reveals a rapid rate of accumulation in the sediments of total organic matter, porphyrin pigments, and hydrocarbons in Santa Monica and San Pedro basins (Emery 1960a, pp. 255, 281). This high rate is due partly to the absence of megascopic benthic animals other than a few vegetative worms (Hartman 1955; Hartman and Barnard 1958, 1960) (Fig. 1). However, certain benthic foraminifera and bacteria do live on the bottom and regen- erate some of the organic materials. When a turbidity-current deposit is formed it

439



440 K. 0. EMERY, JOBST HTLSEMANN, AND K. S. RODOLFO

0 9

yo 4

median diameter-microns--~ / f ^ v s . ., v -a_

Biomoss~~~~~~~~~~ooD

Fi. 1. Genera fetue ofSnaMniaadSnPer ais soter C1aliona o ae Souter iornAext is t, o mfr

9 Sedimentsi a o sn d ite

current and ofclayeyasis deofSted ganic by rain; thed sand-shale ratiosexs the ronal

bottom arena of 0.2uades 0.55 intpas dcriend by Hran (e195 bandshar tanat n Barnardes t (1 9e5,l abundance of the sands (shale is taken as one-half the thickness of the clayey silts in the cores). At the bottom is a map of the distribution of biomass of benthic organisms based upon samples from bottom areas of 0.24 and 0.55 m2 , as described by Hartman (1955) and Hartman and Barnard (1958, 1960 ).

buries the previous sediment surface beyond the effective reach of these and other debris-feeding organisms, so that organic matter and porphyrin pigments are more highly preserved beneath than above the sandy layers (Orr, Emery, and Grady 1958). Turbidity currents have contributed land- derived organic matter to supplement the

marine diet of benthic organisms at many areas, including the Santa Monica and San Pedro basins (Heezen, Ewing, and Menzies 1955; Barnard 1961). Shallow-water benthic foraminifera are also contributed (Bandy, In press).

It seems reasonable that turbidity currents starting from shallow depths may be



INFLUENCE OF TURBIDITY CURRENTS UPON BASIN WATERS 441

TEMPERATURE-C SALINITY-%O OXYGEN-ML/L 10 15 33.50 34.00 0 1 2 3 4 5

200

w~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

g- 400-

z = I-

Wi 600

SILL SAN PEDRO BASIN 800 l l | | J STATION 6864

14 FEBRUARY 1960

. , . . I . , _ , , BOrTT M, , , _ I I I I

FiG. 2. Depth distribution of temperature, salinity, and oxygen content in San Pedro Basin. Similar results were obtained in Santa Monica Basin. The station AHF 6864 is the one at the middle south margin of San Pedro Basin in Figure 3.

able to bring appreciable quantities of oxygen down to the floors of basins whose waters normally are of low oxygen content. Such a replenishment of oxygen may lead to periodic repopulation of an otherwise barren sea floor, followed by mixing of sediment (Emery and Hiilsemann 1962). It should have a major effect in such basins as Santa Monica and San Pedro, whose sills are within the oxygen minimum of the open sea so that the bottom waters are of very low oxygen content (Emery 1960a, pp. 108, 112).

In order to learn more of the nature of bottom waters in these basins, and of the possible influence of turbidity currents upon them, a survey was made aboard the University of Southern California research ship VELERO IV. Appreciation is due the Office of Naval Research for aid provided by contract Nonr 228(17).

BASIN WATER

Eighteen stations were occupied in Santa Monica Basin on 20 and 21 December 1959 and 12 in San Pedro Basin on 13, 14, and 15 February 1960. At each station a Nansen

bottle with a pair of protected reversing thermometers was tripped near the bottom (24 m above the bottom in Santa Monica Basin and 15 m above it in San Pedro Basin). Another Nansen bottle with reversing thermometers was tripped near the 737-m sill depth common to both basins. Special oxygen bottles (Emery and Hiilse- mann 1962) were placed 6 m above and below the bottom Nansen bottle and 6 m below the top one. In addition to the usual titration for chlorinity from which salinity was computed, the water samples were analyzed for phosphate, nitrate, and silicate by spectrophotometer. Suspended sediment was concentrated by centrifuging 50-ml aliquot portions of the water from the Nansen bottles, mounting measured parts of each concentrate on a microscope slide, and counting the inorganic grains of various size groups under a petrographic microscope with a calibrated ocular.

Profiles of water characteristics from surface to bottom were measured at several stations, data from one of which are given in Figure 2. The water temperature de- creases and salinity increases with depth;



442 K. 0. EMERY, JOBST HULSEMANN, AND K. S. RODOLFO

thus the only water from the open sea which can fill the basin below its sill is that at sill depth. Slight gradients of temperature and salinity below the sill probably result from some mixing of water just above the sill depth with that just below; periodic inflow of new water from slightly greater depths than the sill may also be a factor. The de- crease of oxygen with depth below the sill is due chiefly to bacterial and other oxida- tion of organic matter which falls through the water column and lies exposed on the bottom for a time before burial. However, inflow of new water occurs often enough to prevent complete stagnation of the basin water. The rates of oxygen use and nitrate production below the sill depths of other basins in the region are such that complete replacement of basin water in less than two years must take place (Emery 1960a, p. 111). Most of the replacement probably is in the form of movements across the sill.

The grid of temperature measurements made in both basins (Fig. 3) reveals con- centric isotherms for waters near the bottom. At sill depth the waters of Santa Monica Basin are nearly isothermal, but those of San Pedro Basin present a complex pattern for which the best explanation may be disturbance by deep internal waves during the three days of field work. A possible indication of the influence of turbidity currents is given by a slight basinward deflection of the isotherms opposite the mouths of Hueneme and Mugu canyons at the west end of Santa Monica Basin.

The pattern of salinity (Fig. 3) is more interesting than that of temperature, espe- cially for Santa Monica Basin. In the west- ern half of the basin is a large area of salinity much lower than elsewhere. This area is immediately off the mouths of Hueneme and Mugu canyons atop the broad apron of sandy sediments which have been deposited by turbidity currents from these canyons. In the absence of any better explanation we believe that this low-salinity water is a residual indication of a turbidity current which may have occurred shortly prior to the survey. It is notable that the area of low salinity also is one in which the

boundary of low biomass (Fig. 1) is far- thest from the basin side slopes, as though turbidity currents may have permitted benthic organisms to live in depths greater than normal for the basin.

Oxygen exhibits a pattern as simple as that of temperature, with a similarly slight basinward deflection of the isopleths at the west end of Santa Monica Basin. Compari- son of its pattern with that for biomass (Figs. 1, 3) suggests that the limiting oxygen content for an active population of benthic organisms (usually dominated by polychaete worms and ophiuroids) must be about 0.13 mm/L. Nutrient concentrations form a pattern similar to those of temperature and oxygen. In the areas of lowest oxygen content phosphate is in greatest concentration (3.5 to 3.9 ,ug at./L) and de- creases only slightly upward to sill depth. The greatest values of silicates (more than 100 jug at./L) occur in the same areas. Measurements of nitrate were considered unreliable.

Suspended sediment is present in every water sample and increases with depth. In the samples near the basin floor the median diameter (based on the computed volume of each size group) ranges between 10 and 65 ,u (Fig. 4). Significantly, the median diameter of suspended sediment is much coarser than that of the bottom sediments (Fig. 1). Comprehensive studies in various oceans show that suspended materials of several origins are finer than the bottom deposits beneath them (Lisitsin 1961). The median diameter values of the suspended sediment in Santa Monica Basin are not distributed in a decisive pattern. However, the number per milliliter of grains coarser than 62 ,u (all are finer than 125 ,u) is much greater at stations over the turbidity-current apron than elsewhere in the basin (Fig. 4), a pattern similar to that of low salinity (Fig. 3). Expressed in terms of volume, the greatest abundance of suspended material is in the same general area as the coarsest grains. It is considered likely that the areas of concentrated and coarse sediment mark a recent turbidity current possibly still slowly moving, and that the lower concen-



INFLUENCE OF TURBIDITY CURRENTS UPON BASIN WATERS 443

Temperature-?C - at sitt depth-737m, -

Temperature-?C- near the bottom ,;-s-... : _

-I~~~~~~~~~~I

Sahlnity-% at silt depth-737m,/ n~<

Salirsty-%~~~~~~ 3~~034 4

~~~~~~~~~~~\ \ \* SaLinity-%O \ - near the bottom -o

Oxygen-ml/L at siLL depth-737m -?

Oxygen-ml/L 9 near the bottom 1

19 ~ ~ ~ ~ ~ ~ m'

FIG. 3. Areal distribution of temperature, salinity, and oxygen content at sill depth and near the bottoms of Santa Monica and San Pedro basins. Dashed contour is basin sill depth. Surveys were made in December 1959 and February 1960, the latter for San Pedro Basin.



444 K. 0. EMERY, JOBST HUJLSEMANN, AND K. S. RODOLFO

Median Diameter-microns -

4 2 4

41 ~~~~~~~~~~~38

41 19

Grains/mL ~>62,u

Total Suspended Sediment \ ppm by voLume ,--

I//

FIG. 4. Areal distribution of suspended sediment near the bottom of Santa Monica Basin, 21- 22 December 1959.

trations of finer sediment elsewhere in the basin represent background inherited from older turbidity currents.

RECOGNITION OF START OF A TURBIDITY

CURRENT

In order for surveys such as that of Fig- ures 3 and 4 to be most effective, they should be made during or immediately after the passage of a turbidity current. A simple technique would entail setting off a large turbidity current artificially; however, attempts to do this by explosives and sand dumping (Shepard 1951b; Buffington 1961) have failed.

Natural turbidity currents probably move at least once a year through certain submarine canyons off southern California, as indicated by the build-up of sediments at their heads and by the sudden disappear- ance of these sediments (Shepard 1951a). Sounding surveys at the heads of the canyons would have to be repeated almost con- tinuously if a change in bottom depth is to

signal the initiation of a turbidity current. This is not practical. Accordingly, the studies of Figure 3 were made a few days after a minor storm on the thought that this first storm of the season might have caused a mass movement at the head of Hueneme or Mugu canyons. Winds during this storm exceeded 50 km/hr and gale warnings were displayed along the entire coast.

Better methods for recognizing the start of a turbidity current are needed. Some ef- fort has been spent at Scripps Institution of Oceanography to install an electric trip line in the nearby Scripps Canyon; however, maintenance of electrical equipment in the ocean is a serious problem. A simple alter- native was developed using drift bottles. One hundred bottles each carrying a pre- addressed postcard and instructions were divided among 4 stations 22 to 25 m deep at the head of Hueneme Canyon (Fig. 5). Through the aid of divers at U. S. Navy Civil Engineering Laboratory at Port Hue- neme nests of holes were jetted at each station to a depth of about 50 cm. A bottle was placed in each hole and covered by sediment. If the sediment were to become disaggregated by a mass movement or a turbidity current, the bottles should bob to the water surface, drift to shore, be found by bathers, and the authors be notified of their arrival within a few days after their release from the sediment. Previous studies of surface waters in the region using drift cards had resulted in recovery of 46% because of popularity of the beaches; thus good returns could also be expected of these bottles. By March 1962 only 10 bottles had been reported since their place- ment in August and September 1959. One bottle came from station 1, one from station 3, and 8 from station 2. Six of those from station 2 were reported throughout the period May to December 1961 from beaches in southern California; one other was re- covered in February 1962 on a beach 350 km northwest of Port Hueneme (Fig. 5). Although the recovery from station 2 is now considered indicative of a turbidity current, it was not so interpreted earlier because of



INFLUENCE OF TURBBIDITY CURRENTS UPON BASIN WATERS 445

o 2?00 4 00 o

METERS CONTOURS IN METERS

1958 SURVEY

45--

60~~~~~~

5 .0 |3'

120- 119- 118-

FIG. 5. Stations where drift bottles were buried at the head of Hueneme Canyon in August and September 1959. Depths at the stations were some- what shallower than indicated by the contours, which are from a survey made in 1958. Lower panel shows positions where bottles were found along the coast up to March 1962.

the long intervals between the returns.2 In view of the possible use of the method by others, it is described herein even though it did not serve as the signal for a water survey in this region.

CONCLUSIONS

In addition to contributing sediments and shaping sea floor topography, turbidity currents should modify the bottom water in proportion to the quantity of shallow water which is entrained in the sediment-laden flow. This water should produce local anomalies near the bottom in the form of higher temperatures and oxygen contents, and lower salinities and nutrient contents. The contributions of oxygen and commonly of pieces of woody debris may serve to aid benthic life in greater degree than the benthic life is impeded by blankets of coarse sediment of overall low organic content.

A survey to discover and map water anomalies perhaps due to turbidity currents

2 Between March and September 1962, there were 6 more returns all from station 2.

revealed a large area of lower salinity and coarser, more concentrated suspended sediment than elsewhere in Santa Monica and San Pedro basins, and nearby minor de- flections of isopleths of temperature and oxygen content. The areas of low salinity and high temperature, oxygen and suspended sediment are atop an apron of turbidity-current deposits at the mouths of two active submarine canyons. Their posi- tion supports the interpretation that some bottom water was emplaced by a turbidity current which flowed through one of the canyons after a minor storm. Although the evidence is not conclusive, the importance of the phenomenon warrants further investi- gations elsewhere in the world. Continued studies in this area are not worthwhile for several years because longshore movement of inshore sediment is temporarily blocked by recent construction of harbor jetties and a breakwater immediately up-beach from Hueneme Canyon.

REFERENCES

BANDY, 0. L. 1962. Foraminiferal trends asso- ciated with deep-water sands. J. Paleontol., (In press).

BARNARD, J. L. 1961. Gammaridean Amphipoda from depths of 400 to 6,000 meters. Galathea Report, Scientific Results of the Danish Deep- Sea Expedition Round the World 1950-52, 5: 23-128.

BUFFINGTON, E. C. 1961. Experimental turbidity currents on the sea floor. Bull. Amer. Assoc. Petroleum Geol., 45: 1,392-1,400.

EMERY, K. 0. 1960a. The Sea off Southern California: A Modern Habitat of Petroleum. John Wiley & Sons, Inc., New York, 366 pp.

. 1960b. Basin plains and aprons off southern California. J. Geolog., 68: 479-484.

, AND E. E. BRAY. 1962. Radiocarbon dating of California basin sediments. Bull. Amer. Assoc. Petroleum Geolog., 46: (In press. )

, AND J. H-ULSEMANN. 1962. The rela- tionship of sediments, life, and water in a marine basin. Deep-Sea Res., 8: 165-180.

GORSLINE, D. S., AND K. 0. EMERY. 1959. Tur- bidity-current deposits in San Pedro and Santa Monica basins off southern California. Bull. Geolog. Soc. Amer., 70: 279-290.

HARTMAN, OLGA. 1955. Quantitative survey of the benthos of San Pedro Basin, southern Cali- fornia, Pt. 1; Allan Hancock Pacific Expedi- tions, 19(1): 1-185.

, AND J. L. BARNARD. 1958. The benthic fauna of the deep basins off southern Cali-



446 K. 0. EMERY, JOBST HULSEMANN, AND K. S. RODOLFO

fornia. Allan Hancock Pacific Expeditions, 22(1): 1-67.

- AND . 1960. The benthic fauna of the deep basins off southern California, Pt. II. Allan Hancock Pacific Expeditions, 22(2): 69-297.

HEEZEN, B. C., M. EWING, AND R. J. MENZIES.

1955. The influence of submarine turbidity currents on abyssal productivity. Oikos, 6: 170-182.

LIsITSIN, A. P. 1961. Distribution and composi- tion of suspended materials in seas and oceans. In Recent Sediments of the Seas and Oceans:

U.S.S.R. Academy of Sciences, Commission on Sediments, Division of Geological and Geo- graphical Sciences, Moscow, pp. 175-231. In Russian.

ORR, W. L., K. 0. EMERY, AND J. R. GRADY. 1958. Preservation of chlorophyll derivatives in sediments off southern California. Bull. Amer. Assoc. Petroleum Geolog., 42: 925-962.

SHEPARD, F. P. 1951a. Mass movements in submarine canyon heads. Trans. Amer. Geophys. Union, 32: 405-418.

* 1951b. Transportation of sand into deep water. Soc. Econ. Paleontolog. Mineralog., Sp. Publ. 2: 53-65.



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Influence of Turbidity Currents Upon Basin Waters